Analysis of Path-Dependent Damage and Microstructure Evolution for Numerical Analysis of Sheet-Bulk Metal Forming Processes

Sheet-bulk forming processes are applied to manufacture complex components with intricate shape elements or with large variations in wall thickness from sheet metals. Accumulated plastic strains achieved in sheet-bulk metal forming are substantially larger than in conventional sheet metal forming. Differing from sheet forming, the stress state is three-dimensional for these processes due to the thick sheets and process kinematics. Due to these specific process conditions, conventional methods to predict failure in sheet forming such as forming limit curves are not sufficient. Thus, process analysis as well as characterisation of microstructural and mechanical properties for a prediction of properties affecting failure of formed components require other methods. Application of constitutive models for damage computation allows predicting the onset of failure during forming operations. Moreover, even before failure, the mechanical properties, i.e. the elastic stiffness of components, are affected by the evolution of voids. Previous research did not focus on the comparison of different model strategies with respect to the accuracy of predictions and the necessary strategy for parameter identification and validation. This contribution demonstrates that a Gurson-type model, which relied on high-resolution microstructural data, provided the best prediction of failure for a local indentation and sheet upsetting. Suited preparation methods were developed to analyse small voids in the nanometre range. A novel fracture criterion is shown to offer the best compromise of identification effort, implementation effort and accuracy. The assessment of the effect of void evolution on component properties is an important aspect. Different non-destructive methods were validated based on measurements of resonance frequency and propagation velocity. A quantitative relation between the measured void area fraction and the elastic properties was established for components relevant for sheet-bulk metal forming. A testing procedure to determine the performance of components under elevated strain rates was evaluated and the prediction capacity of different modelling approaches with respect to the strain rate sensitivity was compared.